The present invention relates to an apparatus for producing perlite granules and a process for expanding raw perlite to form perlite granules in the updraft.
The heat treatment of inert materials in an oxidizing atmosphere in the updraft, such as the melting and the expansion of perlite particles to form perlite granules, is known.
Because of their properties, perlite granules are used, inter alia, as an insulation material, filtration material or as horticultural granules. During the production, the starting material used is the so-called raw perlite. Raw perlite is a natural glass of volcanic origin which contains 3 to 6% by weight of chemically bound water of hydration. The raw perlite is reduced to grain sizes between 0.125 and 2.24 mm and classified to the grain-size distribution necessary for the properties of the final product.
The raw perlite particles are melted by being heated to temperatures above 1000.degree. C. As a result of the water hydration, which is driven out and evaporated beginning at 800.degree. C., the melted particles expand to a multiple of their initial volume.
Nowadays the vertical kiln, as compared with the horizontally rotating kiln, has largely become widespread as the apparatus for producing perlites.
The vertical kiln comprises a vertical expansion tube made of heat-resistant steel, which is heated from below by a flame. Heat losses as a result of heat radiation are minimized by a refractory insulation, which is fitted at a distance around the expansion tube.
The granular, dried raw perlite is metered directly into the flame in the region of temperatures above 1000.degree. C. through openings made in the periphery of the expansion tube. Depending on the raw perlite grain size, the flame temperature and the residence time, the individual grain is melted and expanded by the evaporating water of hydration. Because of the lift forces resulting from this increase in volume, the expanded particles are carried out upwards by the exhaust gas from the flame. The expanded perlite is separated from the waste gas in a downstream cyclone, extracted by a cellular wheel, cooled and temporarily stored in silos.
The fuel used for the vertical kiln is combustible gases natural gas, propane and butane, as well as heating oil. The fuel is burned hyperstoichiometrically, which means with an excess of air, using air as the oxidation medium.
The maximum permissible operating temperature of the expansion tubes depends on the material. The material generally used is a heat-resistant steel.
The temperature in the furnace is typically between 850.degree. C. and 950.degree. C. During operation, low wall temperatures are aimed at in order to increase the service life, since scaling occurs to an increased extent above 900.degree. C.
The expansion of the raw perlite particles is essentially influenced by the position and the three-dimensional extent of the hot flame zone and the temperature of the latter. The temperature can be influenced by the nozzle geometry of the burner, the fuel/air ratio and by the furnace pressure, that is to say the supply of additional air, but only within specific limits without the permissible operating temperatures being exceeded. The inert nitrogen component in the combustion air, and the excess air in the flame, additionally limit the temperature in the combustion zone and therefore the transfer of heat to the raw perlite particles. As a result, given an existing cross section of the expansion tube, the throughput of expanded perlite granules with a defined bulk density is limited.
Furthermore, the thermal expansion of perlite uses a great deal of energy, having an overall efficiency of less than 30%, depending on the raw particle diameter and the melting point of the raw perlite.
In the existing raw perlite expansion furnaces there is therefore a fundamental interest in improving productivity, that is to say in increasing the throughput of expanded perlite. In addition, the consumption of energy should be reduced.
It is known to increase the throughput of perlite expansion furnaces by enriching the combustion air with oxygen, mixing the combustion air enriched with oxygen together with the fuel gas and adding it through suitable burners into the expansion tube for combustion (U.S. Pat. No. 4,179,264). In this process of oxygen enrichment, which is simple to implement, the throughput of expanded perlite can be increased by a maximum of 30% without the permissible wall temperatures being exceeded.
During the enrichment process, the uniform increase in the oxygen concentration in the primary air simultaneously raises the temperature in the reaction zone, and hence improves the transfer of heat to the raw perlite particles. This means that the throughput of raw perlite can be increased in this way. However, since the location and the extent of the reaction zone or the flame front cannot be changed, the possible increase in the throughput is limited, since the residence time remains constant.
Furthermore, higher oxygen concentrations lead to problems with regard to scaling of the expansion tube and its mechanical strength.